Drum notations are not supported

Most drum notation software use MusicXML as output format, it would be nice if vexflow could handle drum glyphs.

[ringw]

If drum glyphs are added to the upstream VexFlow, I could add MusicXML parsing support for them. As far as I know, they are not currently available. If you don't mind, please open an issue at https://github.com/0xfe/vexflow.

[ringw]

I've just added percussion clef support to the MusicXML parser. I think the remaining steps are to add the "x" note glyph and any other custom glyphs, and then I will also need to add support for those in MusicXML.

Actually, I took a look at Vexflow source code, and percussions are partially handled. You can find some example in the /test/test_percussions.js file. Yet, they don't seem to be implemented in VexTab (or I don't know what I should write to display them), hence my confusion. Moreover, the chosen symbols are not standard.

I will take a look at your modifcation, thanks :)